In an electrophotographic printing machine such as a copying machine, a laser printer, etc., image forming processes are generally carried out in the following manner. By exposing an image formed on a document, toner is made to adhere to an electrostatic latent image formed on a photoreceptor, and after transferring the toner to a transfer sheet, the toner is melted with the application of heat so as to be permanently affixed thereto, thereby forming an image (copying). In such an electrophotographic printing machine, as the above-mentioned image forming process is repeated, the respective properties of the image forming devices including expendables such as a photoreceptor, a developer material, etc., and of a charger deteriorate, and for this reason, a surface potential of the photoreceptor and an amount of toner adhering thereto change which cause variations in copy density and copy brightness, thereby presenting the problem of unstable image quality.
In order to prevent the above problem, the conventional electrophotographic printing machine is provided with an image-quality stabilizer for detecting an amount of toner adhering to the surface of the photoreceptor or a surface potential which affects the amount of adhering toner. This, in turn, executes a feedback control on the image forming devices including a charger, a developer unit, a discharge lamp, and an exposure optical system so as to obtain a constant detected value. More specifically, the image-quality stabilizer executes feedback-controls on the image forming devices so as to stabilize the image quality. Therefore, an image forming device obtained at a reasonable price can be used and an expensive image forming device whose property can be ensured against the repetitive use is not needed. Moreover, an exchange cycle of the expendables such as the developer, etc., can be made longer. Therefore, the described arrangement offers an electrophotographic printing machine which ensures stable image quality at a reasonable price and a low running cost. The feedback control is executed for example, when the electrophotographic printing machine is installed or when the main switch of the machine is turned ON. For example, by executing the feedback control in the pre-rotation of the photoreceptor at the initial start of the copying operation and during subsequent copying operations, the copy density and the copy brightness can be controlled in respective desirable ranges, thereby producing copies with stable image quality.
However, even if unstable image quality ascribable to changes in properties of the image forming devices against the repetitive use can be prevented, if, for example, the image forming device obtained at a reasonable price shows a substantial temperature dependency, and the surface potential of the photoreceptor and the amount of toner adhering thereto change as in the previously described case, variations in copy density and copy brightness occur. Thus, the described arrangement does not give a solution to the problem of unstable image quality. More specifically, the copy density (image density) and the temperature in the copying machine have the following relationship as shown in FIG. 9. At low temperature, the copy density becomes low, while at high temperature, the copy density becomes high as the charging ability of the photoreceptor changes according to the temperature in the copying machine. The variations in copy density can be maintained in a desirable range as long as the charger output is fixed at 400 V, and the temperature in the copying machine is set at 40.degree. C. However, when the temperature of the copying machine changes, the copy density may not be maintained within the desirable range.
In the copying machine provided with image forming devices which have temperature dependencies, if a copying operation is repeated without executing the feedback control on the image forming devices, the temperature in the copying machine may be changed by switching it OFF and ON. According to this change in temperature, the copying density also changes as shown in FIG. 5. At room temperature (20.degree. C.), after the power switch is turned ON, the temperature in the copying machine is heated to 40.degree. C. in about 1.5 hours and according to this temperature rise, the copy density increases to 30%. More specifically, in accordance with the relationship between the copy density and the temperature in the copying machine shown in FIG. 9, when the power switch is turned ON, the temperature in the copying machine is equal to the room temperature (20.degree. C.), and the copy density is 24%, and in 0.5 hours, the temperature in the copying machine is heated to 30.degree. C. and the copy density increases to 27% and falls in an appropriate range of the copy density. Further, when 1.5 hours has passed after the power switch is turned ON, the temperature in the copying machine is heated to 40.degree. C., and the copy density increases to 30%. Namely, when the copying machine is in the OFF state, the temperature in the copying machine is low, and even after the power switch is turned ON, the temperature of the photoreceptor remains low for a while, and thus the copy density is outside the appropriate range, i.e., lower than the appropriate copy density. Thereafter, the temperature in the copying machine is heated by a heat source such as a thermal fuser provided in the copying machine, etc., and accordingly the copying density finally falls within the appropriate density.
In the case where the appropriate copy density range is set to 27-33%, after the power switch is turned ON again, an inadequate image having a low copy density is formed for the first 0.5 hours of operation.
This lowering of the copy density ascribable to the temperature dependency may be prevented, for example, by the following applications:
(1) adopting image forming devices and expendables which show desirable temperature dependencies; PA1 (2) providing a temperature stabilizer in the electrophotographic printing machine; or PA1 (3) setting a greater permissible range for the image quality characteristic. PA1 adhering toner amount detecting means for detecting an adhering toner amount of a reference toner image formed on a photoreceptor; PA1 time measuring means for measuring a set time interval between feedback controls; PA1 time interval altering means for altering the set time interval based on a difference between a detected adhering toner amount and a predetermined reference toner amount at a start of every feedback control; and PA1 control means for executing a feedback control on an output from charger means based on an altered time interval, so that the detected adhering toner amount becomes equal to the predetermined reference toner amount.
However, none of the above techniques prevents the copy density from decreasing because of the following problems.
Namely, in the method (1), the high stabilizing characteristic for the expendables, etc., are required, and a high cost is required. The respective properties of the desired expendables, image forming devices, etc., may not be ensured.
In the method (2), a temperature detector may be required, or a warmer is required for the photoreceptor, and thus the problems of high cost and an increase in power consumption are presented.
In the method (3), the copy density cannot be maintained efficiently, the amount of toner is likely to be excessive or insufficient, and the expendables cannot be used efficiently for a long time.
Therefore, in the conventional copying machine, the feedback control is executed in order to compensate for the changes in the properties of the photoreceptor, the image forming devices as they deteriorate at a predetermined timing set based on time that the copying machine is not in use or the copy count number, etc., so as to prevent unstable image quality ascribable to changes in temperature.
Since the copy density changes gradually as the photoreceptor or the image forming devices, etc., deteriorate, this problem, ascribable to changes in temperature, can be prevented by executing the feedback control at an appropriate timing.
However, even when the copying machine provided with the described image-quality stabilizer is used, if the feedback control is not executed at an appropriate timing, the following problems would occur.
Namely, in the case where each interval between feedback controls is set long, and the next feedback control is not carried out at a desirable time interval as shown in FIG. 10, the photoreceptor is heated after the first feedback control which is to be executed when the power switch is turned ON, and the surface potential increases, and the copying machine is overcompensated. Ascribable to the increase in the copy density, variations in the image quality occur, thereby presenting the problem of increasing an amount of toner consumption as denoted by the slashed area in the figure.
On the other hand, when an interval between feedback controls is set short as shown in FIG. 11, the feedback control is frequently carried out even after the copy density is stabilized. Therefore, although the copy density can be maintained within the desirable range, the problem of increasing the toner consumption occurs. In figures, T.sub.1 and T.sub.3 satisfy T.sub.1 &lt;T.sub.3, wherein T.sub.1 and T.sub.3 may be set at 0.25 hours and 0.75 hours respectively.